Re: [RFC][PATCH 0/4] memcg: add support for hwpoison testing

[Wu Fengguang]

On Tue, Sep 01, 2009 at 03:12:28PM +0800, KAMEZAWA Hiroyuki wrote:
> On Tue, 1 Sep 2009 14:46:52 +0800
> Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:
> 
> > On Tue, Sep 01, 2009 at 10:32:14AM +0800, KAMEZAWA Hiroyuki wrote:
> > > On Tue, 1 Sep 2009 10:25:14 +0800
> > > Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:
> > > > > 4. I can't understand why you need this. I wonder you can get pfn via
> > > > >    /proc/<pid>/????. And this may insert HWPOISON to page-cache of shared
> > > > >    library and "unexpected" process will be poisoned.
> > > > 
> > > > Sorry I should have explained this. It's mainly for correctness.
> > > > When a user space tool queries the task PFNs in /proc/pid/pagemap and
> > > > then send to /debug/hwpoison/corrupt-pfn, there is a racy window that
> > > > the page could be reclaimed and allocated by some one else. It would
> > > > be awkward to try to pin the pages in user space. So we need the
> > > > guarantees provided by /debug/hwpoison/corrupt-filter-memcg, which
> > > > will be checked inside the page lock with elevated reference count.
> > > > 
> > > 
> > > memcg never holds refcnt for a page and the kernel::vmscan.c can reclaim
> > > any pages under memcg whithout checking anything related to memcg.
> > > *And*, your code has no "pin" code.
> > > This patch sed does no jobs for your concern.
> > 
> > We grabbed page here, which is not in the scope of this patchset:
> > 
> >         static int try_memory_failure(unsigned long pfn)
> >         {      
> >                 struct page *p;
> >                 int res = -EINVAL;
> > 
> >                 if (!pfn_valid(pfn))
> >                         return res;
> > 
> >                 p = pfn_to_page(pfn);
> >                 if (!get_page_unless_zero(compound_head(p)))
> >                         return res;
> > 
> >                 lock_page_nosync(compound_head(p));
> > 
> >                 if (hwpoison_filter(p))
> >                         goto out;
> > 
> >                 res = __memory_failure(pfn, 18,
> >                                        MEMORY_FAILURE_FLAG_COUNTED |
> >                                        MEMORY_FAILURE_FLAG_LOCKED);
> >         out:
> >                 unlock_page(p);
> >                 return res;
> >         }
> 
> Hmm. maybe off-topic but why lock_page() is necessary ?

Because we also have filter for testing page flags, which requires
lock_page() to be correct.

> 
> > > I recommend you to add
> > >   /debug/hwpoizon/pin-pfn
> > > 
> > > Then,
> > > 	echo pfn > /debug/hwpoizon/pin-pfn
> > >         # add pfn for hwpoison debug's watch list. and elevate refcnt
> > > 	check 'pfn' is still used.
> > >  	echo pfn > /debug/hwpoison/corrupt-pfn
> > > 	# check 'watch list' and make it corrupt and release refcnt.
> > > or some.
> > 
> > Looks like a good alternative. At least no more memcg dependency..
> > 
> 
> My point is that memcg can show 'owner' of pages but the page may
> be shared with something important task _and_ if a task is migrated,
> its pages' memcg information is not updated now. Then, you can kill
> a task which is not in memcg.

Ah thanks! I'm not aware of that tricky fact, and it does make a
very good reason not to use memcg, although I guess locked page won't
be migrated.

> Then, I don't recommend to use memcg. I think you'll see too much
> pitfalls.

Thanks,
Fengguang
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